There’s a reported fix that works for some, though not 100%:
AppleInsider reported on Monday that a simple procedure has successfully eliminated the bug for many users:
• Disable iMessage in Settings -> Messages
• Reset the iPhone’s Network Settings under Settings -> General -> Reset
• Reenable iMessage.
While the method worked, and continues to work, for most iPhone owners afflicted with the iMessage bug, some have reported that the message failures reappeared after a few hours, forcing them to repeat the steps above.
There’s conjecture that the upcoming 7.03 update will contain this fix. Let’s hope so.
I must be extremely unlucky then, because when I attempt to reenable iMessage, activation fails causing me to have to restart my phone before it works again. iPhone 5, 16GB, iOS 7.0.2.
based on how much has changed in iOS7, I would expect Apple to be quite busy issuing patches for bugs such as this for the next few weeks.
iMessage must default to on, or something. I did not turn it on. I owned an iPad, then an iPhone for years. Never once did I notice the blue vs green bubble thing. As far as I know, I have never used iMessage, but I could be wrong.
Anyway, I “discovered” it when I read about it in the press.
It seems like more trouble than it is worth. I turned it off on all my devices.